It may sound crazy that the Transportation Security Agents assigned to keep guns out of airports have no protection against anyone who brings a gun to a security checkpoint planning to use it. So when a man showed up Friday at Los Angeles International Airport with a rifle, he was able to kill one TSA agent and wound two other agents and another person before he was finally brought down.

The obvious way to prevent a repetition is to either provide guns to security screeners or station armed guards nearby. But the obvious answer is not necessarily the smart one. The shooter in this case was able to start his rampage at the security checkpoint because he knew no one would stop and search him before he got there. If armed guards were normally present in the security area, he might have started shooting sooner.

It may be possible to reduce the chance of a successful attack on TSA agents. But it's not possible to prevent shootings at airports. Someone intent on mass slaughter could open fire at the ticket counters, at the taxi stand or in the parking garage. If you put guards in one place at the airport, the aspiring killer will find another one. Or he'll go to a shopping mall or a crowded street.

The lesson of the LAX shooting is not that we need to spend more money on more security at airports. It's that no security measures will ever be enough.